adding marking on baby milk bottles

If you are worried about ink which I feel should be OK as it's marked on outside, maybe consider changing to other bottles that are clearly marked. Only spectra has this problem actually.

Hmm, try bottles with clear prints. I use sources ppsu bottles, they have both oz and ml markings in red, quite easy to read https://shopee.sg/product/242594539/5732034154?smtt=0.0.9

I dont see an issue. I label my baby's bottles with a sharpie.

Use the ppsu bottle safer and better